This disclosure is directed to an acoustic wave transducer for transmitting acoustic waves. In acoustic well logging procedures, formation properties are obtained by transmission of acoustic waves through formations adjacent to a well borehole. An important property derived from acoustic wave measurements includes the velocity of compression waves. Also, the velocity of shear waves is important. The velocity and attenuation of the Stoneley waves are also important. Compressional and shear waves generated by monopole transducers depend on critical refraction at the bore wall. If the velocity contrasts between the bore fluids and formation are not within specified limits, then neither refracted shear not compressional waves are generated. Specifically, if the shear velocity of the formation is less than the acoustic velocity of the bore fluid, no shear waves are generated. Hence, even though shear waves can propagate through the formation with a characteristic velocity this velocity cannot be measured with monopole transducers. However, multipole sources, e.g. dipole or quadrapole, cause the bore to vibrate in modes that depend on the shear wave velocity of the formation regardless of the relative magnitude of the formation and bore fluid velocities. By using such transducers as sources and receivers, the shear velocity of the formation can be measured.
In general, the lower the frequency content of the waves generated in a bore, the more useful they are for determining formation properties.
Generally, lower frequency waves are able to sample a larger volume of the formation surrounding the well borehole. Further, compressional and shear waves at lower frequencies are not significantly attenuated by casing so that cased wells can be logged in the same manner and with the same procedure as non-cased wells.
Stoneley waves with frequency less than about 5 kilohertz are more affected by formation permeability than are those of higher frequency. Recalling as noted above that certain frequencies are more desirable, many prior art transducers form signals where the signal is dependent on the diameter of the transducer in the sonde. In particular, transducer which have piezoelectric ceramic rings and those having magnetostrictive scrolls have resonant frequencies that depend on transducer diameter. Generally, it is difficult to build well logging transducers of these types with resonant frequencies which are much below 12000 hertz or 12 kilohertz. In light of the size limitations, tool diameter has heretobefore served to limit the resonant frequency. Indeed, a tool diameter which is reduced inevitably establishes higher frequency operating range which is not as useful.
This disclosure, however, sets forth a transducer which can generate single and multipole radiation patterns at low frequencies which are independent of tool diameter. Hence, the apparatus can form low frequency compression or shear refracted waves. Also, low frequency Stoneley waves are formed. Low frequency flexural and screw modes can also be excited by the transducer. Indeed, the description will set forth a transducer which successfully operates at a selected low frequency having a completely independent relationship to the diameter of the tool and the mounting system which supports the transducer.
From the foregoing, certain advantages of the present apparatus will become more readily apparent coupled with the description as set forth below. One especially important feature is the ability to shape the radiation pattern as it is emitted from the transducer. A symmetrically emitted wave form can be generated which radiates outwardly in all directions. By contrast, a dipole can be operated in accordance with the present disclosure which has preferred radiation directions. In both instances, the output frequency is independent of tool diameter so that a particular frequency can be achieved.